Sound generation from a speaker arranged in a space surrounded by an object easily reflecting a sound that not only directly reaches a listener from the speaker, but also indirectly reaches a listener from the speaker after an object reflects sound at least once. Sound indirectly reaching a listener is referred to as a reverb sound. The timing that reverb sound reaches a listener is later than the timing that sound directly reaches a listener from a speaker. Accordingly, when reverb sound occurs, sound directly reaching a listener overlaps reverb sound to make it difficult for a listener to hear the overlapped sounds. Recently, a cellular phone having a water-proof function and hands-free function has been available. When such a cellular phone is used in a space such as a bathroom where reverb easily occurs, reverb sound may make it is difficult for a listener to hear a sound generated from cellular phone. For this reason, suppressing a reverb sound generated from a speaker has been researched (for example, refer to Japanese Laid-open Patent Publication No. 2009-5274).
For example, the acoustic device disclosed in Japanese Laid-open Patent Publication No. 2009-5274 uses a comb filter to filter a digital signal converted from an acoustic signal so as to attenuate or eliminate a frequency component from a particular frequency band.
However, reverb property varies depending on the environment around a speaker. Accordingly, a frequency band to which a comb filter gives attenuation effect sometimes does not match a frequency band in which reverb property is relatively large. When this occurs, even if a comb filter filters an audio signal to be output from a speaker, reverb is not efficiently suppressed. As a result, sound quality of a sound output from a speaker is not improved, and on the contrary, sound quality may be degraded.